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R.D. Caligiuri, Frattura ed Integrità Strutturale, 34 (2015) 125-132; DOI: 10.3221/IGF-ESIS.34.13 125 Focussed on Crack Paths Critical crack path assessments in failure investigations Robert D. Caligiuri Exponent, Inc., 149 Commonwealth Drive, Menlo Park, CA 94025 USA caligiuri@exponent.com A BSTRACT . This paper presents a case study in which identification of the controlling crack path was critical to identifying the root cause of the failure. The case involves the rupture of a 30-inch (0.76 m) natural gas pipeline in 2010 that tragically led to the destruction of a number of homes and the loss of life. The segment of the pipeline that ruptured was installed in 1956. The longitudinal seam of the segment that ruptured was supposed to have been fabricated by double submerged arc welding. Unfortunately, portions of the segment only received a single submerged arc weld on the outside, leaving unwelded areas on the inside diameter. Post-failure examination of the segment revealed that the rupture originated at one of these unwelded areas. Examination also revealed three additional crack paths or zones emanating from the unwelded area: a zone of ductile tearing, a zone of fatigue, and a zone of cleavage fracture, in that sequence. Initial investigators ignored the ductile tear, assumed the critical crack path was the fatigue component, and (incorrectly) concluded that the root cause of the incident was the failure of the operator to hydrotest the segment after it was installed in 1956. However, as discussed in this paper, the critical path or mechanism was the ductile tear. Furthermore, it was determined that the ductile tear was created during the hydrotest at installation by a mechanism known as pressure reversal. Thus the correct root cause of the rupture was the hydrotest the operator subjected the segment to at installation, helping to increase the awareness of operators and regulators about the potential problems associated with hydrotesting. K EYWORDS . Fatigue; Rupture; Ductile Tear; Pressure Reversal; Hydrotest. I NTRODUCTION he Pacific Gas and Electric Company (PG&E) owns and operates an extensive natural gas pipeline transmission system throughout Northern California. One of those pipelines, known as Line 132, is a 30-inch (0.76 m) nominal pipe that transports natural gas up the San Francisco Peninsula to the City of San Francisco itself. The maximum allowable operating pressure (MAOP), as set in accordance with applicable regulations, is 400 psig (2.76 MPa). It typically operates at less than 375 psig (2.59 MPa). The pipeline was originally installed in circa 1948 at a time when the natural gas distribution system in the United States was being built to accommodate demand and transport gas from sources, such as the Permian Basin in the Southwestern United States, to markets, such as San Francisco. In 1956, a subdivision of the City of San Bruno was to be built over a portion of the right-of-way of Line 132. To accommodate the subdivision, a portion of the line known as Segment 180 was relocated. Part of this relocation involved a portion of piping consisting of six short pieces of pipe, known as pups, each approximately 5 to 6 feet (1.52 to 1.83 m) long, joined together. On September 9, 2010, Line 132 ruptured in this assembled pup section. The pressure at the point of rupture had reached 383 psig (2.64 MPa) at the time of rupture but was still below the MAOP. Examination showed that the rupture T